Heretofore, an exposure apparatus has been widely used for transferring a fine circuit pattern onto a wafer to produce an integrated circuit in the lithography technique. According to the progress in achieving high integration, high response speed and high performance of integrated circuits, miniaturization of the integrated circuits has progressed and the exposure apparatuses are requested to form an image of a circuit pattern on a wafer surface with a long focal depth and with high resolution, and use of an exposure light source emitting shorter wavelength has been in progress. As the exposure light source, besides the conventional g-line (wavelength 436 nm), i-line (wavelength 365 nm) and KrF excimer laser (wavelength 248 nm), ArF excimer laser (wavelength 193 nm) is starting to be employed. Further, in order to deal with next-generation integrated circuits having a circuit line width of at most 100 nm, F2 laser (wavelength 157 nm) is regarded as a predominant candidate for the exposure light source, but this light source is considered to cover only until the generation of 70 nm line width.
In such a technical trend, as a next generation exposure light source, a lithography technique using EUV light (extreme ultra violet light) attracts attention since it is considered to be applicable for plural generations of 45 nm and after. EUV light means light in a wavelength band in a soft X-ray region or in a vacuum ultra violet region, and specifically, light having a wavelength of from 0.2 to 100 nm. Currently, as a lithography light source, use of 13.5 nm has been studied. The principle of the exposure in this EUV lithography (hereinafter referred to as “EUVL”) is the same as the conventional lithography in that a mask pattern is transferred by employing a projection optical system. However, since there is no material transmitting light in the energy region of EUV light, it is not possible to use a refraction optical system, and a reflection optical system has to be employed instead. (Refer to Patent Document 1: JP-A-2003-505891)
A mask to be used for EUVL is basically constituted by (1) a glass substrate, (2) a reflective multi-layer film formed on the glass substrate, and (3) an absorptive material layer formed on the reflective multi-layer film. As the reflective multi-layer film, one having a structure that a plurality of materials having different refractive indexes at the wavelength of the exposure light, periodically laminated with the period in the order of nm, is employed, and Mo and Si are known as the typical materials. Further, as the absorptive layer, Ta and Cr are studied. As the glass substrate, a material having a low thermal-expansion coefficient is required so as not to have deformation even under irradiation of EUV light, and a glass having a low thermal-expansion coefficient or a crystallized glass have been studied. The glass substrate is produced by polishing such a glass or crystallized glass material with high precision and cleaning.
The glass substrate to be used for a mask of EUVL having a low thermal expansion coefficient, may, be a synthesized quartz glass substrate doped with e.g. Ti. It has become apparent that when the surface of this glass substrate is mechanically polished by a polishing pad with a free abrasive, local waviness may be formed on the surface of the glass substrate due to local difference in the composition of the material of the substrate. It has become necessary to efficiently remove this waviness to improve the flatness of the surface of a glass substrate.
Patent Document 1: JP-A-2003-505891
Patent Document 2: JP-A-8-120470